This invention relates to a shading correction signal generating device for a television camera apparatus. More particularly, the invention relates to a shading correction signal generating device which is suitable in use for a system in which various types of shadings superposed on picture signals derived from a picture pick-up device are corrected in an analog manner, and the shading correction does not significantly influence the level adjustment at a succeeding stage.
As is well known, the shadings occurring in the picture signals from TV cameras are generally classified into two types. The shading of the first type additively occurs in the picture signals from the image pick-up device. This shading is the pedestal shading due to varied dark current of the pick-up device and varied bias light. The second type of shading occurs in the picture signals in a multiplying manner. This shading makes the black level nonuniform on the entire screen. The shading is called a modulation shading, which arises from, for example, a sensitivity variation due to beam landing error of the image pick-up device, a variation of incident light amount in the optical system, including lenses, and a light-separation optical system, and nonuniform sensitivities of indiviudal picture elements of the solid state image pick-up device. The modulation shading results in an output decrease in the peripheral portion of the screen.
Both types of shadings may be mixed with the true picture signals of a photographed image in the additive and multiplying manner. For the correction of these shadings, waveform correction signals of vertical and horizontal scanning components, usually combined sawtooth and parabolic waveforms, are applied to the picture signals from the image pick-up device in the additive and multiplying manner of analog, to cancel out the shadings.
It is a common practice that, for each correction signal as the base in the correcting process, its AC component along is superposed for correction. The usual way to pick up the AC component is to cut off the DC component contained in each correction signal by a coupling capacitor, for example.
Thus, only the AC component is used for the shading correction. In the correction, the flyback period or blanking period of the vertical and/or horizontal shading waveform must be zero. To this end, the correction waveform is chopped during the blanking period by means of a chopper means. The reason for this is that the blanking period of the vertical and/or horizontal scannings contains the period for providing the pure black level, and this period must be kept intact.
FIG. 4B shows a parabolic waveform, which has been conventionally used, for correcting the superposed shading over one horizontal period 1H. The parabolic waveform is superposed on a picture signal S of FIG. 4A, to obtain a corrected waveform, as shown in FIG. 4C. As seen, the correction signal waveform Sp contains only two points, "a" and "b", where the instantaneous value is zero. For example, the correction waveform has a negative instantaneous value at point "c" which is located at the center of the horizontal period. If such a correction waveform is superposed on the picture signal S of FIG. 4A, the pedestal level in the center area of the screen changes horizontally on the screen. This is correspondingly applied for one vertical period 1V. Further, the same thing is true for the correction for the modulation shading.
Normally, the level adjustment of the picture signals is made for the center area of the TV screen for both vertical and horizontal. Therefore, it is inevitable that the center area level, particularly the pedestal level, changes. A level adjustment for the correction of this pedestal level change has been made in a proper succeeding stage.